Bend rectangle along its long axis
The key point to an optical illusion is that it is an "optical illusion" and you usually can't model them accurately. You should create a model that looks right from the camera, but may not look right as you rotate around the model.

For the triangle you are doing look at this

A tip that can help is to use the Blender Internal for rendering. Under the Options panel of the material there is an Invert Z Depth setting. I haven't tried doing the same in cycles.

This scene -



renders as this -



I believe this was created by Ton back in blender 1.70, you get a copy of the file here